The present invention relates to a ceramic heater device, for example, used as a glow plug for a diesel engine.
In a prior art ceramic heater device, for example, disclosed in a Japanese Utility Model Publication No. 54-95628, a cup-shaped heater body is made of a ceramic material and an open end thereof is attached to a metal housing through a metallized layer formed on an outer surface of the open end. Since in the prior art ceramic heater device of this kind, the open end of the heater body is directly connected to the metal housing, it is disadvantageous in that the heat generated at the heater body may be transferred to the metal housing resulting in that the heat can not be effectively used to ignite the air-fuel mixture in an engine combustion chamber and that a connecting portion of the heater body is used as an electrode whereby the whole portion of the heater body can not be used to generate heat.
It is a further disadvantage of the prior art heater device that heat stress may be generated at the connecting portion which acts as the electrode and through which the heat may be transferred to the metal housing, resulting in that the mechanical and/or electrical connection between the metal housing and the cup-shaped heater body may be deteriorated.
It is a further disadvantage that the specific resistance value of the heater body is decreased since the open end of the heater body is used as the electrode and this open end may not be used as a heating body.